Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element, whereas in other applications the term “valve” includes the moving element, the valve seat, and the housing that contains the moving element and the valve seat. In the following disclosure, a valve suitable for high-pressure abrasive fluids, such as oil field drilling mud, comprises a valve body and seal assembly (the moving element) and a corresponding valve seat.
A valve body and seal assembly typically incorporates an elastomeric seal within a circumferential seal retention groove of the valve body. Such valve body and seal assemblies are commonly found in valves mounted in the fluid end of a high-speed pump incorporating positive displacement pistons or plungers in multiple cylinders (e.g., a mud pump). If preformed seals are to be used, such a groove requires finish machining to closely match the dimensions of seals like the “snap-on” type or seals secured with a removable seal retention plate.
Such groove finish machining may be reduced or eliminated if an elastomer (characterized when cured by a durometer or hardness value and/or a modulus) is cast and cured in the groove (herein “cast-in-place”) to form a single-durometer seal. Further, cast-in-place seals may be mechanically locked to a valve body by forming them over interengaging or interlocking (herein “interdigitating”) projecting-receiving formations on the valve body. Such interdigitation of valve body and seal has become a common structural feature of cast-in-place seals that is difficult or impossible to find in “snap-on” type seals. But manufacture of valve bodies with interdigitating cast-in-place seals has historically involved added costs. These added costs arose because the seals are preferably bonded to a valve body to increase overall integrity. Such bonding creates new problems associated with stress in the seal elastomer.
Even though the manufacturing cost of valve bodies for bonded cast-in-place seals is almost identical to the analogous cost of valve bodies for “snap-on” seals, the added cost of preparing a valve body for bonding increases the cost of the valve to the point that valves with bonded seals have not been competitive on price. The added costs of bonding include cleaning the valve groove of all oil and contaminants, applying a bonding adhesive, and storing the valves in a low-humidity, dust-free environment while the valves await casting, bonding, and curing of the seal material (typically polyurethane) on the valve body. Proper care in these steps may moderate subsequent stress-related damage to the seal and/or the valve body itself (especially the valve body flange).
Valve body flange stresses (and the associated fatigue failures) may be reduced to a limited extent, and valve sealing improved, by a properly-placed elastomeric seal which contacts the valve seat sealing surface evenly on closure (just before contact of the valve body impact area with the valve seat sealing surface). Improper placement of this seal, however, leads to an out-of-round condition that may actually increase leaks and hasten valve failure. Each leak of high-pressure fluid tends to literally wash away a portion of the hardened steel of a valve body and/or valve seat. Multiple and near-simultaneous failures of this kind in web-seat stem-guided valves may give a failed valve body flange the appearance of a wrinkled cupcake paper.
Further, leaks due to poor placement of cast-in-place elastomeric seals often occur secondary to failure of the elastomer adjacent to the special adhesive that bonds the seal to the seal retention groove. If a portion of the seal is tightly bonded to the groove wall, residual shrinkage stress within the seal elastomer will increase as the elastomer cures because the seal as a whole tends to shrink away from substantially radially-extending walls to which it is bonded. Deleterious effects of this residual elastomer shrinkage stress (i.e., elastomer stress that is present even when the seal is not in contact with a valve seat) may be significantly aggravated when combined with the elastomer stresses (see below) which arise when a valve body and seal assembly is closing against its valve seat. The resulting combined elastomer stress tends to reduce the service life of the seal by predisposing it to shear-related fatigue failures (e.g., cracking, tearing and/or extrusion).
Shear-related fatigue failure is common in the portion of an elastomeric seal that would be subject to extrusion as the valve body mates with the valve seat. The requirement that a valve seal make contact with the valve seat just prior to metal-to-metal contact between the valve body impact area and the valve seat sealing surface means that the seal elastomer is subjected to strong pressure forces as the valve is closing. Additionally, since movement of the seal against the valve seat is restricted by friction, by the metal of the valve body impact area, and by the valve seat itself, a portion of the seal elastomer tends to be extruded into the extrusion gap (between the valve body impact area and the valve seat) as the seal slides down the face of the valve seat on valve closing. Thus, a portion of the seal tends to be repeatedly deformed by this extrusion process each time the valve closes. Sliding shear stress due to this deformation combines with the residual elastomer shrinkage stress and pressure shear stress noted above to precipitate premature failures due to stress-related damage.